In such boots, normally of molded plastic material, the upper may comprise several parts: cuff, rear cover, eventually articulated on the base of the shell about a transverse axis. An axis of articulation may, depending on the case, not exist as such, but the upper should flex relatively to the base of the shell at least to a certain extent according to the conditions of use, and this is the equivalent of articulation.
The general direction of the upper, called "the axis of the upper" for the sake of simplicity, is forwardly inclined with respect to the vertical, with which the base of the shell is fast, by a certain angle called "advanced angle." This advanced angle is subject to variation around an average value according to the momentary circumstances of use as a result of the articulation of the upper on the base of the shell. In the assembly, the more the skier is at a high level, and works in a very flexed position, the more he desires a large average advanced angle (static); this could be of the order of 13.degree. to 20.degree. and sometimes could even reach 25.degree., while for less advanced skiers an angle of 8.degree. to 15.degree. is generally deemed optimal. Moreover, depending on the discipline practised and the condition of the snow, the skier desires more or less flexibility in the articulation of the upper on the base of the shell, i.e., facility for variation of the advanced angle under the effect of stresses. Thus a rather pronounced stiffness is preferred for competition, on hard snow or on ice, and sufficient flexibility on powder snow, in order to determine one's bearings, and during descent in order to arrange the musculature and to seek the flat ski position.
These considerations translate into the fact that, more and more frequently, skiers feel the need for boots not only characterized by an average advanced angle corresponding to their personal criteria, but also having a stiffness of the articulation of the upper on the base of the shell which increases as a function of the stress which they exert on the upper, in the direction of comfort and safety of the users.
It has therefore been sought to satisfy these requirements by devices allowing control of this flexion property. Numerous proposals have been made.
Among the devices known in the prior art, certain ones, like those described, e.g., in French Patent No. 2,100,490 or French Patent Application No. 2,416,661, comprise a damping device with a single- or double-acting spring anchored between a central point located in high position on the front of the cuff and a central point located toward the front of the foot on the rigid shell, or fast therewith. Apart from the very unaesthetic appearance of these devices, they are very awkward and exposed to shocks and untimely hooking during skiing maneuvres. They could thus represent a certain danger, and their reliability is quite uncertain.
In French Patent Application No. 2,278,280, there is proposed a device which could play the expected part in an accessory and partial manner. But this involves mainly connecting elements movable between the rigid shell and the cuff itself, confused with what, in the field of interest here, would be a rear cover, because the entry into the boot proceeds from the front. In fact, this device located at the rear of the boot merely makes up, by its flexibility, for the absence of articulation about a transverse axis of the cuff on the rigid shell. The proposed solution is thus outside the technical field of the present invention.
The two preceding devices operate only in or approximately in a vertical longitudinal plane without concerning the important zone which constitutes the bending fold and the kick zone of the foot.
One may further cite, among many others, the contribution of German Published Application No. 3,044,052 which has something in common with the spring device anchored at two points already mentioned above, and that of French Patent Application No. 2,495,901 where a double flexion plate is also anchored at two end points.
An original solution is also proposed by French Patent Application No. 2,342,040 in which an elongated support element is fixed by one part on the stop before binding of the boot and having at its other end a projecting part introduced between two adjustable buckles in position on the boot, for the purpose of limiting the advanced position of the upper of the boot.
In all cases, the proposed solutions, whose very profusion shows the importance and the difficulty of the problem to be solved, present at least one and generally several major inadequacies: poor or questionable effectiveness of the device, complexity leading to prohibitive cost, limitation only of the projection or maximal flexion of the upper, eventual adjustment made once and for all before the run, causing a lack of progressiveness in its effects and inadequacy with respect to the actual conditions of such run.